An inorganic board has been produced by steps of: molding a hydraulic inorganic material such as siliceous raw material, calcareous raw material or cement into a plate shape, curing/hardening the plate to make a base material, and calcining the base material to form an inorganic board. This inorganic board is excellent in durability, beauty and texture, which allows the board to be used as a building material for a posh external wall. For example, JP 2005-194143A, discloses a method of producing an inorganic board comprising the steps of: molding a board using slag and calcium hydroxide as a hydraulic inorganic material, curing/hardening the board, and then calcining the board. Also, JP 2005-0194144A discloses a method of producing a three layered calcined substance made of a raw material containing slag and calcium hydroxide. The base material is glazed before calcining. However, when generally well-used flit glaze is applied onto a base material containing a lot of vitreous material such as slag and vitreous material, the base material may be corroded by a B2O3 component of the flit or a glass phase with low melting viscosity formed in a relatively low temperature range.